Two-cycle engines have one or more cylinders which generally comprise a crankcase, called a pump crankcase, associated with each of the cylinders, with the crankcase communicating with one of the ends of the cylinder and ensuring introduction of fresh gas into the cylinder through at least one passageway and a transfer opening. The piston, which moves in reciprocating fashion in the cylinder, also ensures the intake and compression of fresh gas in the pump crankcase. An intake valve, mounted on the pump crankcase, allows fresh gas to be introduced into the crankcase when the piston moves in the direction opposite to the crankcase, with the fresh gases then being compressed and ensuring closing of the valve when the piston moves in the direction of the crankcase. When the corresponding openings of the cylinder are uncovered by the piston, fresh gases are introduced into the cylinder through the passageways and transfer openings, producing fresh-gas scavenging designed to replace the combusted gases which are discharged through exhaust openings generally arranged in a slightly offset fashion relative to the transfer openings. The piston moves away from the crankcase, compressing the gases contained in the cylinder. Ignition and burning of the mixture thus produce the displacement of the piston toward the crankcase.
In French Patent 2,496,757, it has been proposed to provide pneumatic fuel injection into the cylinder by using the pressure of the fresh gases inside the pump crankcase. To this end, a means for metering the liquid fuel is connected directly to the passageway coming from the pump crankcase. The air compressed in the pump crankcase, transported to the metering device through the passageway, ensures atomization and injection of the fuel inside the cylinder.
Certain improvements have been made to this device and it has been proposed, for example, to provide a container in the passageway connecting the pump crankcase to the injector and a valve at the end of the passageway connected to the pump crankcase. This therefore constitutes a supply of compressed air at a pressure near the maximum pressure in the pump crankcase during the cycle, with the compressed air supply thus serving to atomize the fuel and introduce the atomized fuel into the cylinder in the form of a fuel-air mixture, when the injector is triggered.
One of the disadvantages of two-cycle engines is the loss of power due to the fact that the fresh carburetted gases are not sufficiently separated from the combusted gases inside the cylinder and are consequently responsible for creating conditions unfavorable for the start of combustion.
To overcome this disadvantage, it has been proposed to install a flow-restricting device in the transfer passageways of the engine, in the vicinity of the cylinder, to slow down the emptying of the combusted gases. This results in stratification of the fresh and combusted gases, with the fresh gases being pushed back into the area of the cylinder where injection and ignition take place. However, no such technique has ever been used to restrict the flow of air coming from the crankcase, in the case of a two-cycle engine comprising a pneumatic injection device using the compressed air from the pump crankcase.
In addition, in the case of pneumatic injection using compressed air from the pump crankcase, the pressure differential between the air used for injection and the gases filling the cylinder at the moment of injection can be generally too low to ensure good atomization of the fuel or very high injection efficiency. This drawback remains significant when a container connected to the injector is used, as well as a passageway separated by a valve from the pump crankcase. The pressure in the container, which is higher than the maximum pressure in the pump crankcase, is sometimes insufficiently higher than the pressure in the cylinder at the moment of injection when the cylinder pressure undergoes an increase caused by wave effects in the exhaust pipe.